Network analyzers are instruments that characterize networks. The characterization result is known to those of ordinary skill in the art and are based on conventions that define how the network will perform under various conditions. In signal integrity applications, the common network parameters in use are scattering parameters, or s-parameters. S-parameters define port to port relationships in a network when the network is driven by a source whose impedance is set to the reference impedance and all other ports are terminated in that same reference impedance. This convention allows scattering parameters to completely define the behavior of a network under any other driving and termination conditions.
Standard instruments for s-parameter measurement include the vector network analyzer (VNA) and the time-domain reflectometer which uses techniques of time domain reflectometry (TDR). These instruments stimulate a network with incident waveforms and measure reflected waveforms at the network ports.
In the measurement of s-parameters, measurement instruments are calibrated to a reference plane. This is a location either at the ports or beyond the ports of the instrument and is usually a coaxial environment and there is a known relationship to electric and magnetic waves at the ports. These coaxial environment ports are useful connection points to the instrument because there are industry agreed upon (upper frequency dependent) standards for cables and connectors. Examples include the subminiature version A (SMA) for frequencies up to 18 GHz and the 2.92 mm or so-called K connector for frequencies up to 40 GHz.
While coaxial connectors offer a convenient, standardized connection to measurement instruments, the DUT that is being measured often does not have these connectors at its measurement ports (ones that do are so-called connectorized devices). Non-connectorized DUTs are frequently encountered in signal integrity (SI) applications where the ports of the DUT might be printed circuit board (PCB) pads or traces. In these applications measurement engineers generally employ either probes or fixtures for interfacing a DUT to the measurement instrument. These probes and fixtures must be of high quality so that the electromagnetic waves propagate between the DUT and the measurement instrument with minimal degradation. Despite their potential high quality, they present unwanted error in the measurement. It is desirable to therefore remove their effects from the measurement thereby isolating the measurement of the DUT.
There have been many methods employed for isolating DUT measurements from measurements containing unwanted error due to connection elements. These methods are loosely separated, but related, into three categories: calibration, de-embedding, and gating.
Calibration is the most complex and varied method. When used as a tool for removing fixture and probe effects, it involves duplicating the process generally used to calibrate the instrument at its usual reference plane. Methods are numerous including short-open-load-thru (SOLT), line-reflect-match (LRM), thru-reflect-line (TRL) and many others including variations on the aforementioned ones. Calibration involves the process of performing multiple measurements of standards whose characteristics are either fully known or can be inferred from the multiple measurements. When calibration is utilized for isolating a DUT measurement in the context here, the one or more standards are placed at the end of fixtures or probes and as a result of the calibration, the measurement reference plane is moved to the tip of the probe or the end of the fixture where the DUT is then connected.
There are many drawbacks, however, to these methods beyond even their general complexity. One is the requirement for multiple measurements which can be time consuming and prone to error. Another is the requirement for the standard elements themselves whose characteristics must be somehow measured or modeled. In the case of probes, generally an impedance standard substrate (ISS) is utilized. These are expensive and multiple, identical probe connections must be made to the substrate for the multiple measurements involved in the calibration. In the case of fixtures, usually multiple structures are utilized where the intent is for the structures to contain the standards coupled with paths with identical characteristics from the standard to the ports whereby the identical path characteristics are identical to the final path from measurement port to DUT port. The drawback here is again identifying the standard characteristics, but also maintaining the identical nature of the path characteristics. Also, any fixture used must generally have these standards on board.
De-embedding is mathematically complex but once this complexity is overcome, it presents other practical problems. It involves constructing a system model containing known fixture and/or probing elements and unknown DUT measurements and then solving for the unknown DUT. U.S. patent application Ser. No. 12/418,645, filed Apr. 6, 2009 to P. Pupalaikis et al., entitled “Method for De-embedding Device Measurements” deals with de-embedding efficiently. The drawback of de-embedding methods is that they still require knowledge of the characteristics of the fixture and probing elements which are often difficult to ascertain. In other words, although the de-embedding math operation can be a final step in determining the s-parameters of the DUT, some method of determining the s-parameters of the probe or fixtures to de-embed is still required.
The final method, gating, generally involves separating portions of responses generally in the time-domain. This method has the benefit of being easy to employ, but suffers from an inability to easily produce a full set of DUT s-parameters. Since s-parameter measurements are made not only for qualitative and quantitative measurements of DUT quality but also for generation of complete device models, gating cannot generally satisfy the latter requirement. Furthermore, gating methods generally do not account for a variety of effects. For example, the time-domain separation of responses does not account for reflections that occur in the measurement due to the gated out portions. There are some methods that exist that combine features of time-domain analysis and de-embedding that are referred to as gating methods (as the present method is sometimes referred to) such as described in J. Dunsmore, N. Cheng and Y. Zhang, “Characterizations of asymmetric fixtures with a two-gate approach,” 77th ARFTG Digest, 2011, pp. 138-143 and D. Metzger, “The Mathematics of Time Domain Substitution,” http://www.constantwave.com but the former does not account for multiple reflections and uses the time-domain impulse response and neither deals with the difference between calibration measurement conditions and the DUT measurement conditions. In the first case, the goal is to analyze the fixture for de-embedding and, once the s-parameters are known, relies on the fixture conditions being substantially similar in the fixture measurement phase and the DUT measurement phase. In the second case, a ratio is performed between a calibration and DUT measurement phase that also includes the reliance on similarity of conditions of the probe and fixture in both calibration and DUT measurement phase.
A general problem that occurs with all of these methods is the maintenance of constant characteristics at the DUT measurement ports. This means that if calibration methods are employed with probes, the structure at the location where the probe tips touch down must be identical in the calibration environment as in the final measurement environment. This is difficult to maintain.
What is needed is a method that overcomes the drawbacks stated.